In addition to the colors on the wheel, you can make use of neutral colors -- black, white, and gray -- and earth tones such as brown, tan and olive to create certain effects.  All make good backgrounds to accentuate other colors.  Here, a dark background makes the foreground appear lighter.  Lighter backgrounds will appear to darken blooms in the foreground.

Here, the dark grass, earth, and shadow in the foreground accentuates the lighter foliage of the shrub.

You can use neutral colors as "punctuation" in the garden. Here, gray foliage plants and white flowers are useful to help separate combinations of plants using different color schemes.

White flowers or foliage with white variegation are especially good for lightening up dark, shady areas.

Yellows and light tints of other colors do the same.

This pachysandra with variegated green and white leaves lightens up deep shade under a tree.

next: Introduction to Color